`.only(..)` does not check eagerly if the fields exists

[Willem Van Onsem]

Django's QuerySets are lazy, but the checks for (most) methods are eagerly, I think. For example:

us = User.objects.filter(bla='qux')

will raise an error.

But only(..) seems to be an exception on this. If we use:

us = User.objects.only('bla')

it will only raise if we evaluate us (enumerate, aggregate, str/len/...).

It seems nice that .only(..) would check eagerly if the fields are available, otherwise tests might succeed if the queryset is in some cases not evaluated.

A possible problem might be that .only(..) could strictly speaking mention fields that are not *yet* defined, but later in the chain of ORM method calls.

[Tim Graham]

Thanks for the suggestion, however, even if feasible, I doubt this is worth the complexity it would likely add, especially considering that defer() and only() are only for advanced use cases.